A method for removing contaminant material from a soil site has been described in U.S. Pat. No. 5,074,986 issued to R. F. Probstein et al. on Dec. 24, 1991. In the method described in the Probstein patent, one or more source electrodes are positioned at one or more first locations in the soil site and one or more sink or collector electrodes are positioned at one or more second locations therein. Voltage gradients are established among the source and sink electrodes and a purging liquid is externally supplied to the source electrode(s) and flows therefrom into the soil site. The purging liquid thereupon moves by electroosmosis through the soil site from the source electrode(s) thereby displacing the contaminant material so as to cause it to be moved into the sink electrode(s) from which it is in turn removed from the site.
One technique for electrode placement that was considered therein is to use parallel electrode arrays where a plurality of source electrodes and sink electrodes are arranged in alternating rows throughout the soil site. Because of the relatively large numbers of electrodes required and because the contaminant material must be collected from so many sink electrodes, the manifolding thereof is complicated and relatively costly so that such an arrangement, though effective, is believed to be economically inefficient for many practical situations.
Alternatively, it has been suggested that an effective placement pattern of electrodes in a soil site for soil dewatering and consolidation processes would include the providing of a plurality of source electrodes which are positioned so as to surround a single sink electrode (or a small number of closely spaced sink electrodes). Such an electrode configuration pattern might be considered as desirable in a contaminant removal process in order to make the removal process more effective since the number of sink electrodes can be considerably reduced, often to a single one, and placed at only one relatively small region of the site. Accordingly, the need for a large number of removal channels is eliminated and the cost thereof is substantially reduced. The use of such a pattern, which provides an inwardly radial flow from the source electrodes to the sink electrode, is discussed, for example, in connection with soil dewatering and consolidation processes in the text "Fundamentals of Soil Behavior" by James K. Mitchell, John Wiley and Sons, New York, 1993, 2nd Edition, pp. 256-285.
However, analysis of such a pattern for use in a contaminant removal process of the type described in the Probstein et al. patent shows that relatively large areas of the soil site are left uncleaned, severely limiting the contaminant removal efficiency for the overall site. Moreover, there is no control over what flows into the region so that further contaminant may flow therein from outside the site region and/or the soil may dewater to such an extent that flow may cease, either temporarily or permanently, thereby interrupting the removal process. It is desirable, therefore, to provide a different more satisfactory electrode arrangement which produces the most effective removal of contaminant material from an entire soil site.